build/gmodule-2.0.pc: Move compiler flags from Libs to Cflags

Previously, `-Wl,--export-dynamic` was in `Libs` key of `gmodule-2.0.pc`,
even though `-Wl` is a compiler flag, rather than a linker one.
This caused issues with API reference builds in evolution-data-server,
which passes the output of `pkg-config --libs` through `--ldflags`
argument of `gtkdoc-scan`, which are forwarded unchanged to `ld`:

    ld: unrecognized option '-Wl,--export-dynamic'

Let’s move the flag to `Cflags` so that the compiler can deal with it.

https://gitlab.gnome.org/GNOME/evolution-data-server/-/issues/463

gmodule/meson.build

@@ -115,22 +115,24 @@ pkg.generate(libgmodule,
   description : 'Dynamic module loader for GLib',
 )
 
-pkg.generate(libraries : [libgmodule, export_dynamic_ldflags],
+pkg.generate(libraries : [libgmodule],
   requires : ['glib-2.0'],
   version : glib_version,
   variables : [supported_var],
   install_dir : glib_pkgconfigreldir,
   filebase : 'gmodule-export-2.0',
+  extra_cflags : export_dynamic_cflags,
   name : 'GModule',
   description : 'Dynamic module loader for GLib',
 )
 
-pkg.generate(libraries : [libgmodule, export_dynamic_ldflags],
+pkg.generate(libraries : [libgmodule],
   requires : ['glib-2.0'],
   version : glib_version,
   variables : [supported_var],
   install_dir : glib_pkgconfigreldir,
   filebase : 'gmodule-2.0',
+  extra_cflags : export_dynamic_cflags,
   name : 'GModule',
   description : 'Dynamic module loader for GLib',
 )